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Hackers are exploiting a vulnerability in the popular vBulletin Internet forum software in order to inject rogue administrator accounts into websites using it.

The exploit was found by researchers from security firm Imperva on underground hacker forums and targets versions 4.x.x and 5.x.x of vBulletin.

At the end of August, vBulletin Solutions, the company that develops the forum software, advised users to delete the "install" directories from their vBulletin deployments because of an unspecified exploit vector.

The company declined to release any additional information about the issue at that time, but Imperva's researchers believe it's the same vulnerability targeted by the exploit script they found.

The vulnerability allows attackers to abuse the vBulletin configuration mechanism to create a secondary administrative account, the researchers said Wednesday in a blog post.

In order to exploit the vulnerability, attackers need to know the exact URL for the upgrade.php script from the install directories of the targeted vBulletin deployments and the vBulletin customer IDs associated with those deployments.

To obtain this information, hackers created a separate PHP script that scans vBulletin sites for the vulnerable path and extracts the customer IDs from the source code of the upgrade.php pages, the researchers said.

Once they have those details, attackers only need to choose a username and password for the rogue administrator account that will be created and the exploit will do the rest.

VBulletin Solutions declined to confirm whether the exploit identified by Imperva is the one they warned about in August.

"We've released updates to vBulletin 4 and vBulletin 5," said Wayne Luke, technical support lead at vBulletin Solutions, Wednesday via email. "These are vBulletin 4.2.2 and vBulletin 5.0.5. We recommend customers delete their install directories when not in use maintaining their software."

"I cannot provide any further information on the issue," Luke said.

Notes accompanying the exploit script found by Imperva claim the vulnerability was discovered on Aug. 22.

The Imperva researchers have seen traffic from an attack that successfully exploited a forum powered by vBulletin 4.2.0 and there are also reports about successful attacks against others using version 4.2.1, said Tal Be'ery, security research team leader at Imperva, Thursday via email. However, the company hasn't tested whether the latest 4.2.2 and 5.0.5 versions are vulnerable to the exploit, he said.

The directories that vBulletin users should delete in order to protect their deployments are "/install" for vBulletin 4.1.x versions and "/core/install" for the 5.x versions.

Users who, for some reason, are unable to delete these directories, can use the Web server access configuration mechanism or a Web application firewall to block requests or redirects for upgrade.php, the Imperva researchers said.

The vBulletin software powers over 100,000 Internet community forums, including some very large ones. UbuntuForums.org, a community website for Ubuntu Linux with more than 1.8 million registered accounts that was compromised by hackers in July, is also using vBulletin.